1. Field of the Invention
Aspects of the present invention relate to a support for a fuel reforming catalyst, and more particularly, to a support for a fuel reforming catalyst with excellent heat and mass transfer characteristics to be able to exhibit higher activity properties even when containing the same quantity of catalyst as a conventional support, and a method of preparing the same.
2. Description of the Related Art
Fuel cells form an energy generating system in which the chemical energy of oxygen and hydrogen contained in hydrocarbons such as methanol, ethanol, and natural gas is directly converted into electric energy.
In general, a fuel cell system has a basic structure of a stack, a fuel processor (FP), a fuel tank, a fuel pump, and the like. The stack forms the body of the fuel cell and has a stacked structure of a few or tens of unit cells, each including a membrane electrode assembly (MEA) and a separator or bipolar plate. A fuel pump supplies a fuel contained in a fuel tank to a fuel processor. The fuel processor reforms and purifies the supplied fuel to generate hydrogen and supplies the hydrogen to the stack. In the stack, the hydrogen and oxygen electrochemically react to generate electrical energy.
FIG. 1 is a flow chart illustrating stages of processing a fuel 30 in a fuel processor 10 of a conventional fuel cell system. Referring to FIG. 1, a desulfurization process, a reforming process 20, and a CO removing process are performed in the fuel processor 10. In particular, the CO removing process may include a high-temperature shift reaction, a low-temperature shift reaction, and a preferential oxidation (“prox”) reaction. Using these processes, the fuel 30 is generated, and the generated fuel 30 is supplied to the stack.
A reformer, which is used in the reforming process 20, reforms a fuel formed of hydrocarbon using a reforming catalyst. It is increasingly likely that the hydrocarbon will be methane, because a liquefied natural gas mainly formed of methane is expected to be a prominent feedstock of fuel cells in the future. In the reforming process, steam (H2O) is added to the methane to produce hydrogen through Reaction Scheme 1:

The Reaction Scheme 1 occurring in the fuel reforming process is an endothermic process that requires a great amount of heat. Accordingly, the reforming process requires a supply of heat.
Meanwhile, a rate-determining step, which determines the overall rate of reaction, can be a reaction step, a heat transfer step, a mass transfer step, or an adsorption/desorption step. In the fuel reforming, the rate-determining step is a heat transfer step or a mass transfer step. Accordingly, in order to increase the entire reaction rate, it is important to increase the heat transfer rate and the mass transfer rate.
Pores formed in a catalyst support are categorized into micropores, mesopores, and macropores, according to the pore size. According to the definition of pores set by the International Union of Pure and Applied Chemistry (IUPAC), a micropore has a size less than 2 nm, a mesopore has a size of 2-50 nm, and a macropore has a size greater than 50 nm. Meanwhile, in some cases, a pore having a size of 10-100 μm is defined as an ultrapore though such a definition is not set by the IUPAC. In this specification, however, pores having a size of 10-100 μm are also regarded as macropores.
When the proportion of micropores or mesopores is high, a relatively large surface area of the catalyst support can be obtained, which disadvantageously makes mass transfer slower. Accordingly, when the rate-determining step is a reaction step, a high proportion of micropores is advantageous.
When the proportion of macropores is high, the surface area becomes smaller, but faster mass transfer can be obtained. Accordingly, when the rate-determining step is the heat transfer step or the mass transfer step, a high proportion of macropores is advantageous. However, when the proportion of macropores and mesopores is substantially small, the entire surface area is excessively reduced and thus the subsequently manufactured supported catalyst is less active.
When a reaction is endothermic, like a fuel reforming process, and the heat transfer or mass transfer controls the overall reaction rate, micropores, mesopores, and macropores need to exist in a proper ratio. However, a support having such a pore distribution has not yet been developed. Conventional supported catalysts have structures formed mainly of macropores, or formed mainly of micropores and mesopores.
In order to facilitate heat transfer, the surface of a metal can be encapsulated with a metal oxide acting as a support. However, the thickness of the formed metal oxide is typically too small, and an adhesive force between the metal and the metal oxide is weak.